The W-CDMA scheme may experience multipath interferences due to higher chip rates. Techniques for reducing such multipath interferences include equalizers such as FIR filters (see Non-Patent Documents 1 and 2 that will be described later, for example), and the CCMRM (chip correlation minimum mean square error (MMSE) receiver with multi-path interference correlative timing (MICT)) (see Non-Patent Document 3 that will be described later, for example).
These techniques determine weighted-combined coefficients (tap coefficients) such that interference components are effectively canceled using the correlation between paths and such that the signal component is maximized. In order to eliminate the interferences, reception data in the vicinity of the signal (path) timing within an uniform-interval timing of one chip is used.
For example, an equalizer (FIR filter) is configured to include n delay circuits (where n is an integer equal to or greater than 2) that delay reception data incrementally by an unit time (one chip), and n tap multipliers that multiply the outputs from the delay circuits by tap coefficients w1, w2, . . . , wn (hereinafter, also referred to as w), and an addition circuit that adds (determines the total sum of) the outputs from the tap multipliers. The reception data is delayed at each of the delay circuits, thereby forming n data series including non-delayed reception data, and the data series are multiplied with tap coefficients w, which are added (summed) at the addition circuit.
The above-described tap coefficients w are determined by resolving simultaneous linear equation Eq. (1.1) using the correlation matrix R (having i rows by j columns) representing the correlation between paths and channel estimation values h.
                                          ∑            j                    ⁢                                    R                              i                ,                j                                      ⁢                          w              j                                      =                  h          i                                    (        1.1        )            
In addition, in the CCMRM, when determining the correlation matrix R, the average of the product of signals d prior to despreading (one of signals di+k, dj+k the path timing of which are shifted each other by (i−j) samples is multiplied as a conjugate complex number to the other of di+k, dj+k, as expressed by the following Eq. (1.2). Note that in Eq. (1.2), the symbol “*” represents a conjugate complex number and k represents the path number.
                              R                      i            ,            j                          =                              ∑            k                    ⁢                                    d                              i                +                k                            *                        ·                          d                              j                +                k                                                                        (        1.2        )            
The above Eq. (1.2) can be expressed in the following Eq. (1.3) using the channel estimation values h and the noise estimation value n in an equalizer:
                              R                      i            ,            j                          =                              ∑            k                    ⁢                      (                                                            h                                      i                    +                    k                                                  ·                                  h                                      j                    +                    k                                                              +                                                n                  2                                ⁢                                  δ                                      i                    ,                    j                                                                        )                                              (        1.3        )            where δi,j represents the Kronecker delta.
Here, the channel estimation values h and the correlation matrix R for the predetermined window width (the number of tap coefficients) is to be determined, and this window width includes timing other than the targeted signal (path) (desired signal) timing in order to eliminate the multipath interference. In addition, the channel estimation values h can be determined by despreading the common pilot signal.
Note that other conventional techniques for reducing multipath interferences include the techniques described in the Patent Documents 1 to 4 listed below.
The technique in Patent Document 1 relates to a spectrum spread RAKE receiver, in which the path timing of a multipath is detected, the path timing is set as the despreading timing. For the delay time between arbitrary two paths, centering around one of the path timings, two timings of the other path timing and the target location are set as the despreading timings for all of the combinations of any two paths. For each timing that is set, a despreading signal for a reception signal is determined and each of the despreading signals is combined.
The technique in Patent Document 2 is a technique for eliminating interferences from an adjacent base station. The interference reduction timing τ±(τ2−τ1) for eliminating the interference component incoming from the adjacent base station is determined as well as determining the path timing τ of a multipath from a base station with which communication is carried out. The interference component is reduced by despreading the received spectrum-spread signal by setting these timings as despreading timings τ, τ±(τ2−τ1) and weighting-combining the despreading results.
The technique in Patent Document 3 achieves a RAKE receiver having the MIXR (Multipath Interference exchange Reduction) function in a realistic hardware scale even numeral paths are detected. Among path timings detected by a path search and the MICT generated from the path timings, timings are selected in number that is equal to the number of fingers, which is provided to the fingers. Upon selecting the timings, an expected value of the SNIR after the RAKE synthesis is calculated, and the path timings and the MICT are selected such that that value is maximized.
The technique in Patent Document 4 relates to a RAKE receiver that can effectively reduce the interference noise even if the number of paths is increased. Upon reception of direct spectrum-spread signals of N paths, the reception timing ti (where i=1 to N) of each of the N paths is detected. The reception signal is despreaded by using, as a despreading timing, timing ti,j,k (where k=1 to N, k≠j) that gives the despreading value having a correlation with the interference from the jth (j=1 to N, j≠i) path which is within the despreading value of the ith path from this reception timing ti and synthesizing the despreading signals.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-133999
Patent Document 2: Japanese Laid-Open Patent Publication No. 2005-354459
Patent Document 3: Japanese Laid-Open Patent Publication No. 2005-94134
Patent Document 4: International Patent Publication No. WO2004/025860
Non-Patent Document 1: A. Klein, “Data Detection Algorithms Specially Designed for the Downlink of Mobile Radio Systems”, Proc. of IEEE VTC'97, PP. 203-207, Phoenix, May 1997
Non-Patent Document 2: 3GPP R4-040680, “HSDPA improvements for UE categories 7 and 8”, Nokia, TSG RAN WG4#33, November 2004
Non-Patent Document 3: Hasegawa, Shimizu, “A Study on Sign Bit Chip Correlation MMSE Receiver with MICT,” Proceedings of the IEICE General Conference, B-5-8, 2006
Although the channel estimation value at a timing without signal should be zero in theory, the above-described conventional techniques may provide a channel estimation value being the noise component even at a timing without signal since despreading is performed to determine the channel estimation value. As a result, the tap coefficient calculation accuracy by an equalizer may be deteriorated, which may degrade the reception characteristic.
Note that although each of the techniques described in the Patent References 1 to 4 adopt various ways to set despreading timing aiming at obtaining interference reduction effects, they fail to eliminate the timing of the noise component. In addition, the techniques cannot solve the above-identified problems because they determine channel estimation values in a conventional manner.